1. Field of the Invention
This invention relates to an device for and method of maintaining a constant distance between a cutting edge, e.g. focal point of a cutting laser beam, and a reference surface, e.g. surface of a coating on a first major surface of a sheet, to impose a cut or separation line of uniform width in the coating, and more particularly, to impose a cut or separation line having along its cut path a constant width at a constant distance from the reference surface.
2. Discussion of the Technical Problems
As is appreciated by those skilled in the art, laminated aircraft windshields can have one heating arrangement on inner surface of the outer ply, e.g. outer glass sheet, of the laminated windshield to prevent the formation of ice on outer surface of the windshield, and can have a second heating arrangement between structural plies, e.g. glass or acrylic sheets to prevent and/or to remove fog from inner surface of the windshield. The heating arrangement usually includes an electrically conductive coating, e.g. one or more coating films between a pair of bus bars connected to an electrical power source. The boundary of the conductive coating usually is spaced from the edges of the sheet and corresponds to the peripheral shape of the sheet.
One of the limitations with the presently available heating arrangements is that their use on sheets that have a non-linear or curved periphery prevents uniform distribution of the current through the electrically conductive coating that has a non-linear or curved peripheral shape. The non-uniform distribution of the current through the coating can result in hot spots in the coating, which can result in overheating of the windshield, which can result in de-lamination of the windshield. One technique to eliminate the hot spots is to segment the electrically conductive coating to provide for a more uniform current flow through the coating. The width of the cut or separation lines imposed in the coating to segment the coating is sufficiently small, e.g. but not limited to a width of 0.0015 inches (0.048 millimeters (hereinafter also referred to as “mm”)) such that the operator of the aircraft looking through the windshield has minimal or no visual observation of the separation lines, and the width of the separation lines is sufficiently large such that the separation lines electrically isolate the segments from one another in the area between the bus bar.
One technique to segment the coating is to make one or more passes of a laser beam over the coating. More particularly, the focal point of the laser beam is positioned to impinge on the conductive coating at a predetermined spaced distance from a reference surface of the sheet to impose a separation line in the coating by evaporating a portion of the coating as the laser beam moves along its path. Although this technique, is acceptable for electrically conductive coatings applied to flat surfaces of sheets, there are drawbacks when the technique is practiced on coatings applied to curved surfaces of sheets. One of the drawbacks is that the distance between the focal point of the laser beam and the reference surface of the sheet varies as the surface curvature of the sheet varies. The result is that the width of the deletion line increases as the distance between the focal point of the laser beam and the reference surface decreases and vice versa. One approach to eliminate this drawback is to develop a software program to follow the contour of the curved surface of the sheet. As is appreciated by those skilled in the art, the contour of the curved surfaces of two glass sheets is not identical, requiring a software program for each sheet, which is time consuming and expensive. Another approach to eliminate the drawback is to place a member in surface contact with the surface of the coating to displace the focal point of the laser beam as the surface curvature changes. The drawback with this approach is that the member can mar the surface of the coating as the member moves the focal point of the laser beam in response to changes in the curvature of the surface of the coating.
As can now be appreciated by those skilled in the art, it would be advantages to provide a technique for imposing one or more cut lines or separation lines in a coating, e.g. an electrically conductive coating, that has a generally constant width at a constant distance from a reference surface and does not have the drawbacks of the presently available techniques.